The present invention relates to a vapor source for focused ion beam systems and particularly to such a source which is compatible with corrosive materials.
Focused ion beam systems are used in forming, shaping or altering microscopic structures in semiconductor devices, or in other solid materials, including semiconductor or insulating surfaces and metal areas providing electrical connection. The focused ion beam can be directed to a very small point on a semiconductor device and then scanned, raster fashion, over a surface where material is to be removed. As an ion impinges on the semiconductor device surface, its momentum is transferred resulting in the removal of one or more surface atoms according to a process called sputtering. By selecting a raster pattern of a given overall shape, for example a horizontal raster pattern, a correspondingly shaped area of surface material can be removed. Often several successive layers of a semiconductive device are removed in a given area in order to reach and possibly sever an underlying layer.
It has been discovered that the above described physical sputtering process can be enhanced by the introduction of a chemically reactive gas. The gas is adsorbed on the surface of the semiconductor device prior to arrival of the ion beam and the gas reacts chemically with the surface material to produce an area of material that is more easily sputtered away. Thus, a chemical reaction takes place with the surface material resulting in easily removed compounds. Not only is gas enhanced sputtering faster, but also is less subject to redeposition of sputtered material. One focused ion beam system employing gas enhanced sputtering is described in U.S. Pat. No. 5,188,705 to Swanson et al entitled "METHOD OF SEMICONDUCTOR DEVICE MANUFACTURE". In this particular system, solid iodine is heated within a crucible for producing iodine gas that is directed toward the sample being sputtered.
Since sputtering enhancement materials are highly chemically reactive, they tend to corrode dispensing apparatus, shortening the lifetime thereof and creating potential leak problems. The typical vapor source employs a mechanically operable valve for regulating gaseous flow to the sample. However, moving surfaces tend to function improperly and stick after a relatively short lifetime. Feedthroughs for valve actuators, for instance including a sliding seal, tend to become nonfunctional with time. Metal bellows can be used as a mechanical feedthrough in a vacuum wall for the purpose of avoiding the sliding seal and providing protection against gas leaks. However, such bellows are not generally available in a material compatible with the aforementioned corrosive gases and moreover contain folds where corrosive substances can become trapped.